fix tail trimming of switch blocks (#1707)

now guarded under `dead_code`

fixes #1705

lib/compress.js

@@ -2509,7 +2509,7 @@ merge(Compressor.prototype, {
             var expression = make_node_from_constant(value, self.expression).transform(compressor);
             self.expression = best_of_expression(expression, self.expression);
         }
-        if (compressor.option("dead_code")) {
+        if (!compressor.option("dead_code")) return self;
         var prev_block;
         var decl = [];
         var body = [];
@@ -2531,6 +2531,7 @@ merge(Compressor.prototype, {
                     if (default_branch) {
                         body.splice(body.indexOf(default_branch), 1);
                         extract_declarations_from_unreachable_code(compressor, default_branch, decl);
+                        default_branch = null;
                     }
                 } else if (exp !== branch.expression && !fallthrough) {
                     extract_declarations_from_unreachable_code(compressor, branch, decl);
@@ -2559,24 +2560,22 @@ merge(Compressor.prototype, {
             body[0].body = decl.concat(body[0].body);
         }
         self.body = body;
-        }
-        while (branch = self.body[self.body.length - 1]) {
+        while (branch = body[body.length - 1]) {
             var stat = branch.body[branch.body.length - 1];
             if (stat instanceof AST_Break && compressor.loopcontrol_target(stat.label) === self)
                 branch.body.pop();
-            if (branch.body.length
-                || branch instanceof AST_Case
-                    && branch.expression.has_side_effects(compressor)) break;
-            self.body.pop();
+            if (branch.body.length || branch instanceof AST_Case
+                && (default_branch || branch.expression.has_side_effects(compressor))) break;
+            if (body.pop() === default_branch) default_branch = null;
         }
-        if (decl && self.body.length == 0) {
+        if (body.length == 0) {
             return make_node(AST_BlockStatement, self, {
                 body: decl.concat(make_node(AST_SimpleStatement, self.expression, {
                     body: self.expression
                 }))
             }).optimize(compressor);
         }
-        if (body && body.length == 1 && (body[0] === exact_match || body[0] === default_branch)) {
+        if (body.length == 1 && (body[0] === exact_match || body[0] === default_branch)) {
             var has_break = false;
             var tw = new TreeWalker(function(node) {
                 if (has_break

test/compress/switch.js

@@ -614,3 +614,69 @@ issue_1698: {
     }
     expect_stdout: "2"
 }
+
+issue_1705_1: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        var a = 0;
+        switch (a) {
+          default:
+            console.log("FAIL");
+          case 0:
+            break;
+        }
+    }
+    expect: {
+        var a = 0;
+        switch (a) {
+          default:
+            console.log("FAIL");
+          case 0:
+        }
+    }
+    expect_stdout: true
+}
+
+issue_1705_2: {
+    options = {
+        dead_code: true,
+        evaluate: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 0;
+        switch (a) {
+          default:
+            console.log("FAIL");
+          case 0:
+            break;
+        }
+    }
+    expect: {
+    }
+    expect_stdout: true
+}
+
+issue_1705_3: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        switch (a) {
+          case 0:
+            break;
+          default:
+            break;
+        }
+    }
+    expect: {
+        a;
+    }
+    expect_stdout: true
+}